It is well known that yeast strains belonging to the genus Saccharomyces are capable of fermenting sugars to approximately equal amounts of carbon dioxide and ethanol under anaerobic conditions. The leavening activity of yeast in dough is a result of this activity. The rate of carbon dioxide and ethanol evolution in dough is dependent upon, besides the activity of enzymes involved, the sensitivity of the yeast against the amount of sugar present in the dough. In sugar-rich doughs the leavening activity of the yeast may be much lower than in lean (nonsugared) doughs, caused by a higher osmotic pressure related to the presence of the sugar.
The commercial product baker's yeast is produced in several formulations comprising fresh yeast, dried yeast and frozen yeast. Fresh yeast is available as compressed yeast (27-33% dry matter content) and cream yeast (17-23% dry matter content). Dried yeast is available as active dry yeast (ADY) and as instant dry yeast (IDY) with moisture contents of 6-8% and 3-6%, respectively. Improving the fermentative power of yeast is an ongoing research effort. Both the dried yeast and the moist yeast may be improved to increase their carbon dioxide producing ability in lean (non-sugared) as well as in rich (sugared) doughs so as to reduce leavening time and/or enable the use of less yeast, a considerable cost factor in baking.